This invention relates to industrial gas-cleaning systems of the type wherein dry dirty gas, i.e., dry gas with particulate solids entrained therein, passes into an enclosed bag house, flows through the fabric bag filters suspended in the house to remove the solids from the gas with the cleaned gas being exhausted from the bag house.
Typically, industrial bag house systems have two rows of individual bag houses. In a suction system the dirty gas from an industrial process is delivered by a common manifold to all of the bag houses. The cleaned gas discharges from the bag houses into another common manifold which connects to the inlet of a high-capacity blower. The blower maintains a low pressure in the system to induce the desired flow of the gas therethrough, and the clean gas discharges from the blower outlet either to atmosphere or to some further operation.
In a pressure system, the dirty gas passes through a blower and is forced through a common manifold into each bag house. The cleaned gas then discharges from the house, usually to atmosphere.
In either system, dust or other particulate matter will accumulate on the bag filters as the cleaning process continues, gradually reducing the filter efficiency. Periodically, normal flow of dirty gas through the house is interrupted and clean gas is forced into the bag house to flow in a reverse direction through the filter bags to dislodge the accumulated dust. The gas used for the bag-cleaning operation is usually referred to as "reverse air" even though such gas may not be atmospheric air. After the bags are thus cleaned, the flow of reverse air is discontinued and the bag house is put back on stream.
In a multiple bag house installation, continuous operation of the gas-cleaning system is maintained by cleaning the houses one by one while the dirty gas flows through the remaining houses. As a consequence a separate manifold system must be provided to supply reverse air individually to the bag houses. Valving arrangements must be provided to isolate each bag house from the dirty gas flow system and to connect them to the reverse air duct. Timer-operated automatic control systems are provided to operate the various valves so that the bag houses are cleaned in the desired sequence.
Industrial gas-cleaning units are often of very large size and expensive to install. For example, a multiple bag house system may have two rows of ten bag houses, with an overall length of 100 feet, each bag house having a hundred 30-foot-high filter bags suspended therein. The dirty-gas and clean-gas manifolds for such a system may each have a cross-sectional flow area of about 50 square feet. The connections from the bag house to the manifolds and the valves therein may be several feet in diameter.
Because of the size and weight involved, much of the manifold and ducting system must be fabricated at the site, particularly the valved distribution systems which connect the individual bag houses to the dirty-gas, clean-gas and reverse-air manifolds. The separate reverse-air manifold system must be connected to the bag houses in the same general location as the clean-gas or dirty-gas manifold systems are connected thereto, depending on whether a suction or pressure system is involved. In either case, having to provide two separate manifold systems in essentially the same location usually requires a complicated and expensive design in order to fit all the ducting in without sacrificing flow efficiency. The problem is even more acute in a pressure system since the reverse-air manifold system must be fitted into a space already filled with structural supports and the system which removes accumulated dust from the bag house hoppers.
As mentioned previously, valves must be provided to connect the bag houses to the various ducts for normal gas-cleaning or for bag-cleaning. Oftentimes, because of the complexity of the manifold and ducting systems and the large size of the component parts, the valves for a single bag house must be located at physically separated points such that it is difficult or impossible for a single workman to inspect all valves associated with a single bag house at the same time and see that they are operating in the proper sequence.
In instances where the gases to be cleaned are hot combustion products from the burning of coal having a high sulphur content, the system must be designed to maintain the temperature in the gas flow system and the bag houses sufficiently high at all times to prevent condensation and formation of sulphuric acid in the system. In particular, the reverse air must be heated so that when it is injected into a bag house it will not cool the gas therein and cause undesired condensation. The same consideration is involved whenever the gases to be cleaned are hot mixtures of air and water vapor. The reverse air must be above the dew point to prevent undesired condensation.
For hot gas operation in suction houses the source for reverse air is typically the clean-gas manifold from the bag houses since the clean gas therein will already be at elevated temperature. The reverse air taken from the cleangas manifold passes through a blower and is blown through the reverse-air manifold system back into the bag houses. To prevent heat loss, the reverse-air manifold system is usually insulated. If this is not enough to maintain the reverse air at the desired high temperature, an additional heater must be provided for the reverse-air system.
Since the clean-gas manifold must be insulated to prevent condensation therein, the typical hot-gas-cleaning system thus requires separate, and costly, insulation of both the clean-gas and reverse-air duct systems and perhaps an additional heater for the reverse-air system.